Subcell limiting: Revise order of bounds using a Dict

[bennibolm]

In later PRs, the order of bounds in variable_bounds can be very confusing.
For instance, in the branch with all subcell limiting features, there is this code to get the right position in variable_bounds (see here):

@unpack variable_bounds = limiter.cache.subcell_limiter_coefficients

counter = 2 * length(limiter.local_minmax_variables_cons) + spec_entropy +
              math_entropy
if local_minmax
    if variable in limiter.local_minmax_variables_cons
        for (index_, variable_) in enumerate(limiter.local_minmax_variables_cons)
            if variable == variable_
                var_min = variable_bounds[2 * (index_ - 1) + 1]
                break
            end
        end
    else
        for variable_ in limiter.positivity_variables_cons[1:index]
            if !(variable_ in limiter.local_minmax_variables_cons)
                counter += 1
            end
        end
        var_min = variable_bounds[counter]
    end
else
    var_min = variable_bounds[counter + index]
end

This PR now revises the implementation using a Dict, reducing the linked code to 2 only lines:

@unpack variable_bounds = limiter.cache.subcell_limiter_coefficients
var_min = variable_bounds["$(variable)_min"]

The reason why it was so complicated earlier was, that I only allocate the bound arrays for variables which are actually limited. Moreover, if a variable is limited with both the positivity limiter and the local limiter, only the more restrictive bound should be saved, but both limiter should be able to access the saved bound. Therefore, I had to check for every positivity variable, whether the same variable is limited locally. In both cases, the location of the bound in variable_bounds is different.

[github-actions]

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (1727b07) 79.30% compared to head (a45389e) 93.23%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1649       +/-   ##
===========================================
+ Coverage   79.30%   93.23%   +13.93%     
===========================================
  Files         419      419               
  Lines       34165    34143       -22     
===========================================
+ Hits        27093    31833     +4740     
+ Misses       7072     2310     -4762     

Flag	Coverage Δ
unittests	93.23% <100.00%> (+13.93%)	⬆️

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Files	Coverage Δ
src/solvers/dgsem_tree/containers_2d.jl	96.63% <100.00%> (ø)
src/solvers/dgsem_tree/subcell_limiters.jl	83.87% <100.00%> (ø)
src/solvers/dgsem_tree/subcell_limiters_2d.jl	94.34% <100.00%> (-0.10%)	⬇️

... and 148 files with indirect coverage changes

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[bennibolm]

After @efaulhaber 's suggestion from above, I directly implemented the variable variable_bounds as Dict. It actually wasn't necessary to use an additional bounds ordering variable. Your suggestions/questions seem to treat an old version. @sloede

I will push the change to use Symbols instead of Strings and request a review from you.